One class of hybrid electric vehicle powertrains, commonly referred to as a powersplit powertrain, has two sources of power. The first source includes an internal combustion engine, and the second source includes a combination of an electric motor, a generator and a battery. The engine and the generator, together with a planetary gear set, a countershaft and a motor, establish a mechanical torque flow path and an electromechanical torque flow path to vehicle traction wheels. The battery is an energy-storing device for the generator and the motor. Engine power is divided into two power flow paths at any generator speed and vehicle speed. Engine speed is controlled by the generator, which implies that the engine speed can be decoupled from the vehicle speed within the allowed speed range of the generator. This mode of operation is called “positive power split”, when the generator is generating electrical power using mechanical power input from the engine.
Because of the mechanical properties of the planetary gear set, the generator can distribute power to the planetary gear set to drive the vehicle. This mode of operation is called “negative power split”. The combination of a generator, a motor and a planetary gear set thus can be considered to have electrical continuously variable (e-CVT) transmission characteristics.
A generator brake can be activated so that engine output power is transmitted with a fixed gear ratio to the torque output side of the powertrain through a mechanical path only. The first power source can only effect forward propulsion of the vehicle since there is no reverse gear. The engine requires either generator control or application of a generator brake to transmit output power for forward drive.
When the second power source is active, the electric motor draws power from the battery and drives the vehicle independently of the engine for both forward drive and reverse drive. The motor may also generate power and charge the battery if the engine produces power exceeding driver demand, or in a regenerative mode capturing vehicle kinetic energy. In addition, the generator can draw power from the battery and drive against a one way clutch on the engine power output shaft to propel the vehicle in a forward direction. This mode of operation is called “generator drive mode”. A vehicle system controller coordinates the two power sources so that they work together seamlessly to meet a driver's torque demand without exceeding powertrain system limits. The vehicle system controller allows continuous regulation of engine speed for any given vehicle speed and power request. The mechanical power flow path provides efficient power delivery through the planetary gear set to the driveshaft.